It is well known in the machining arts that cutting tools perform best when urged into contact with a workpiece at a specific speed or within a specific range of speeds. Although the particulars of the speed range may vary with workpiece composition or workpiece attributes such as hardness or ductility, this behavior is generally observed. In particular it is observed in both metal and ceramic workpieces, for tool steel, carbide, coated carbide and ceramic tools, and for cutting tools of specified geometry such as milling cutters as well as for tools comprising a bonded assemblage of a more or less randomly-oriented cutting edges such as diamond or ceramic grinding tools.
Many rotating cutting tools, such as mills, burrs and drills, mount the cutting edges at their periphery. Thus as the tool diameter is reduced to enable the creation of smaller features in the workpiece, commonly termed micro-machining, the tool is required to rotate faster to maintain the preferred peripheral cutting speed range since the linear velocity is given by the product of the angular velocity and the tool radius.
For purpose of illustration only, a reasonable value for the preferred cutting speed of aluminum is about 75 meters (about 246 feet) per minute. Thus a rotary tool with a radius of about 500 micrometers (0.5 millimeter or about 0.02 inch) should be operated at a rotational speed of about 25000 revolutions per minute (rpm). Reducing the tool diameter to about 50 micrometers (about 0.002 inch) leads to a tool rotational speed of 250,000 rpm. With a still further reduction to about 25 micrometers (about 0.001 inch), it would necessitate a tool rotational speed of about 500,000 rpm for the cutting tool were to operate in the preferred range.
Thus micro-machines capable of micro-machining must, for robust cutting performance, operate at significantly higher rotational speeds than conventional machine tools. More specifically, for machined features 100 micrometers (about 0.004 inch) in width or less the micro-machine should be capable of operation at several hundreds of thousands of rpm which poses significant challenges in the manufacture and operation of such devices.
As the size of the machined feature shrinks the need for high precision in the micro-machine increases. For example control of tool runout to micrometer levels is required placing stringent requirements on tool-micro-machine attachment systems and on machine spindle alignment and runout among other issues. Since the machine spindle will be supported on bearings, many of the required micro-machine features promote a need for innovative bearing designs.
In turn the machine spindle and bearings must be assembled into a support structure, or housing. It may therefore be important that the housing, bearing and spindle design be consistent with assembly practices which assure high precision in the assembled micro-machine. The assembly practices should be robust, that is, accepting of normal part or component tolerances and assembler skill level, without significant prejudice to performance. The assembly practices should also enable disassembly and reassembly without significant prejudice to performance.